Method for obtaining a bright zinc coating by electrodeposition and the bath used therefor



United States Patent 3,317,412 METHOD FOR OBTAINING A BRIGHT ZINC COAT- ING BY ELECTRODEPOSITION AND THE BATH USED THEREFOR Helmut Dahlmann, Stuttgart, Germany, assignor to Schering Aktiengesellschaft, Berlin, Germany No Drawing. Filed July 3, 1962, Ser. No. 207,426 Claims priority, application Germany, July 6, 1961,

Sch 29,958

5 Claims. (Cl. 204-55) The present invention relates to a method for obtaining a bright zinc coating in the electrodeposition of zinc from an alkaline bath.

The conventional baths for obtaining bright zinc coatings have a cyanide content amounting to about 80 to 150 g./l. of sodium cyanide. The removal of the poisonous cyanides from the rinse water and sewage as required by law and health regulations can be carried out only at a very great expense, and the required safety measures render a zinc plating by means of cyanide baths so expensive that in many cases it can no longer be carried out economically.

Although alkaline zinc baths which are free of cyanide and therefore do not have the mentioned disadvantages are also known, such baths, aside from having a lower electrolytic efliciency than cyanide baths, also do not produce any bright zinc deposits as are attained by cyanide baths. Zinc plating is, however, increasingly in demand and for many purposes it is of a much greater value than coatings of other materials.

According to the invention, it has been discovered that a bright zinc plating of a perfect quality can also be attained if a bath having an alkaline base is provided not only with the usual admixtures of the conventional brighteners and other additives, but also with an addition of alkanolamines. The important advantage of such a bath is the fact that it is free of cyanide and therefore complies with the legal requirements. Furthermore, it is economical in use and simple in application. Especially suitable for this purpose are such alkanolamines as triethanolamine, diethanolamine, triisopropanolamine, and the like. Good results may also be attained with other amines, such as hexamethylenetetramine,which may be applied either as a further addition or by itself. The mentioned compounds may be added, for example, in amounts of about 4 g./l. to 60 g./l. and more.

The other admixtures of the bath according to the invention, for example, brighteners, agents for preventing voids, other kinds of metals, protective colloids, and others are well known in cyanide zinc baths. These include, for example, piperonal, vanillin, anisaldehyde, salicylic aldehyde, gelatin, dextrin compounds of molybdenum, chromium, mangnese, selenium or tellurium, surface-active compounds, and the like.

The bath according to the invention further contains the usual amounts of a zincate which may be produced by dissolving zinc or a zinc compound (for example, zinc oxide, zinc sulphate, or zinc chloride) in alkalis, such as free alkali metal hydroxides. The precipitation of the bright zinc coatings is carried out at room temperature and up to about 45 C. and at a current density of about 1 to 4 a./dm. and in the course of the electrolysis the. mentioned admixtures are added as desired. The plating process is carried out in the conventional manner either by agitating the electrolyte or by moving the material to be plated or by changing the polarity.

Although they are not intended to limit the scope of the invention, the following examples will illustrate several preferred embodiments thereof.

Example 1 40 g. of zinc oxide together with g. of sodium hydroxide are dissolved in 1 liter of water, and hereto are added: 0.8 to 2.0 g. of white gelatin, dissolved in water; 0.2 to 0.4 g. of piperonal, dissolved in alcohol; and 10 to 20 cc. of triethanolamine.

If voids are formed, a wetting agent may be added, for example, one based on fatty alcohol sulphonates in an amount of 2 to 10 g./l.

The electrolysis is carried out at 2 a./dm. and 1.2 to 1.5 v. and at room temperature. The plating period amounts to about 10 minutes. The precipitation amounts to about 0.5 ,u/min. The cathode is provided in the form of iron plates. When the gloss diminishes, it may be regained by the addition of brighteners. The glossy deposits are of an excellent quality.

Example 2 14 g. of crystallized zinc sulphate together with 230 g. of sodium hydroxide are dissolved in 1 liter of water, and hereto are added:

G. Gelatin 1 Pliperonal 0.24 Vanillin 1 Chromium chloride 0.4 Triethanolamine 25 The electrolysis is carried out in the so-called Hull cell at 4 a./dm. and 2 to 3 v. and at a temperature of 20 to 25 C. Within a medium current density range, zinc deposits are attained ranging from bright to glossy.

Example 3 The following compounds are dissolved in 1 liter of water:

From the galvanic bath which is operated at 1 to 3 a./dm. and 1.6 v. at room temperature, glossy zinc deposits are formed.

Example 4 From an electrolyte which contains the following components dissolved in 1 liter of water, glossy perfect zinc coatings are attained at 1 to 3 a./dm. and 1.4 to 1.6 v. and at a room temperature of 18 to 25 C.:

Under the mentioned conditions, excellent glossy zinc deposits are attained with an electrolyte which contains the following components dissolved in one liter of Water:

G. Zinc sulphate, crystallized 70.0 Sodium hydroxide 180.0 Molybdenum oxide 8.0 Piperonal 0.4 Wetting agent based on fatty alcohol sulphonates 2.0 Gelatin 0.8 Triethariolamine 25.0 Hexamethylenetetramine 32.0

Although my invention has been illustrated and described with reference to the preferred embodiments thereof, I wish to have it understood that it is in no way lim ited to the details of such embodiments or to the specific examples described, but is capable of numerous modifications within the scope of the appended claims.

. Having thus fully disclosed my invention, what I claim 1. A method for obtaining a bright zinc coating which comprises exposing the object to be coated as a cathode in an aqueous cyanide-free alkaline zinc bath consisting essentially of a zincate selected from the group consisting of zinc and zinc compounds dissolved in an alkali metal hydroxide solution and an amine, and passing a current in said bath from anode to said cathode.

2. The method of claim 1, wherein said alkaline zinc bath also contains at least one additive selected from the group consisting of the conventional lustering and wetting agents therefor and said amine is selected from the group consisting of the alkanolamines, hexamethylenetetramine and mixtures thereof.

3. A method as defined in claim 1, wherein said amine is employed in an amount of between approximately 4 to about 60 grams per liter of alkaline-zinc bath.

4. A cyanide-free alkaline zinc bath for use in the electrodeposition of bright zinc coatings consisting essentially of an aqueous alkaline solution containing a substance selected from the group consisting of zinc and zinc compounds and an alkali metal hydroxide, and an additive therefor consisting essentially of an amine selected from the group consisting of the alkanolamines, hexamethylenetetramine and mixtures thereof, the ratio of said amine to said solution being from about 4 grams per liter to about 60 grams per liter.

5. A cyanide-free alkaline zinc bath for use in the electrodeposition of bright zinc coatings as defined in claim 4, wherein said additive further includes conventional lustering and wetting agents therefor and wherein said alkanolamines are selected from the group consisting of triethanolamine, diethanolamine and triisopropanolamine.

References Cited by the Examiner UNITED STATES PATENTS Handbook, Reinhold Publishing Corp., page 214, 1955. The Metal Industry, Nov. 29, 1940, pages 433-436.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD FOR OBTAINING A BRIGHT ZINC COATING WHICH COMPRISES EXPOSING THE OBJECT TO BE COATED AS A CATHODE IN AN AQUEOUS CYANIDE-FREE ALKALINE ZINC BATH CONSISTING ESSENTIALLY OF A ZINCATE SELECTED FROM THE GROUP CONSISTING OF ZINC AND ZINC COMPOUNDS DISSLOVED IN AN ALKALI METAL HYDROXIDE SOLUTION AND AN AMINE, AND PASSING A CURRENT IN SAID BATH FROM ANODE TO SAID CATHODE. 